1. Field of the Invention
The present invention relates to an islanding detection method, and more particularly to a detection method that prevents an occurrence of an islanding operation.
2. Description of Related Art
Distributed energy or renewable AC power generating equipment is often operated parallel with AC power system. If the AC power system breaks down due to abnormality and the distributed energy AC power generating equipment cannot detect the abnormal breakdown of the AC power system remain continues supplying power, then an independent power supplying condition occurs. During the independent power supplying period, both the voltage and frequency are unstable, so that other electric appliances or equipments may be damaged easily. In addition, accidents or electric shock may occur due to the negligence of maintenance people in related regions, or the AC power system cannot be synchronized during a power recovery, thus damaging the related equipment.
If the voltage and frequency of the AC power system at the point of common coupling (PCC) are detected and the voltage or frequency goes beyond (either higher or lower than) the operating range, then the distributed power system will be disconnected to avoid the occurrence of an islanding operation. Alternatively, the voltage phase of the power system is detected all the time, and a substantial voltage phase jump indicates the occurrence of an islanding operation. The foregoing voltage, frequency, phase jump methods are usually limited by the form of a load, and sometimes the occurrence of an islanding operation cannot be detected due to the conditions of the load. Such an undetectable zone is known as a non-detection zone (NDZ).
Another technology is to detect the total harmonic distortion (THD) of the power distribution at the point of common coupling (PCC). Distortion exceeding a predetermined standard indicates the occurrence of an islanding operation phenomenon. The exciting current of a transformer varies non-linearly according to the theory of the non-linear B-H curve of the transformer. If a power system operates normally, the non-linear current is supplied by the power system, and thus will not affect the harmonic of voltage, but if the power system breaks down, such non-linear current must be supplied by the distributed power system. At that point, harmonics of voltage appear, and thus the harmonic of voltage can be used for determining an islanding operation. Further, the principle of the direct communication method by using power line carrier is clear and obvious, but it is necessary to install expensive communication equipments on both sides. The foregoing methods are generally referred to as passive islanding detection methods.
Another important detection method is categorized as an active islanding detection-method. U.S. Pat. No. 5,493,485 discloses a way of determining the phase shift between output current and voltage using a factor such as the phase between current and voltage, a frequency of voltage change, an amplitude of voltage change, or a change of total harmonic distortion (THD) of voltage. Such a method is very complicated and difficult to implement into actual practice. So that when a change of frequency of voltage is detected, the phase angle between current and voltage is adjusted to make the operation point to reach another stable point according to the curve. The stable point falls beyond the normal operating zone, and such a curve is shown in FIG. 2.
U.S. Pat. No. 5,111,377 uses a current frequency of a disturbance input power system, and such disturbance has no significant effect upon the voltage frequency when a power system is operating normally. If the power system breaks down, the disturbance will directly affect the voltage frequency and/or the phase difference of the current. When there is a phase difference or a frequency change, the variation will be positively fed back to the disturbance of the current frequency, so that the disturbance is increased again until the voltage frequency exceeds the operating range, so as to detect an islanding operation phenomenon. U.S. Pat. No. 6,172,889 also has similar system architecture.
The method adopting a disturbance input power system also has NDZ as disclosed in U.S. Pat. No. 6,429,546. In other words, when a load is specific, or a phase shift produced by a low-pass filter precisely offsets the original target variation, the voltage or phase change cannot be detected, and thus the islanding operation phenomenon cannot be detected, either. U.S. Pat. No. 6,429,546 uses a polynomial or a forgetting function to give weights to the variation, but such method is complicated and not easy to implement into actual practice. Many of the aforementioned methods use a positive feedback method to adjust the frequency of the voltage or the current, or the phase angle between voltage and current of renewable generation system, and the adjustment is determined mainly by an error. The smaller the error is, the less the adjustment, and the larger the detection time. If an error approaches or equals zero due to a circuit/load delay (lead), then the islanding operation status cannot be detected.